docs: def convertToRateMatrix(self, Q):
          """
          Converts the initial matrix to a rate matrix.
          We make all rows in Q sum to zero by subtracting the row sums from the diagonal.
          """
          rowSums             = Q.sum(axis=1).getA1()
          idxRange            = np.arange(Q.shape[0])
          Qdiag               = coo_matrix((rowSums,(idxRange,idxRange)),shape=Q.shape).tocsr()
          return Q-Qdiag